External fixation systems used in bone surgery are intended to maintain the interfragmentary setting during the consolidation phase of the fracture or the osteotomy. These systems provide a mechanical connection with osseous levers and the outside medium. This orthopedic device comprises one or several fixation bars connected to at least two pins by means of clamps, said pins being fixed by known means to the bone fragments in the case of a fracture.
At present, the means principally used in medicine to evaluate the consolidation of a fracture are radiography and manual evaluation of the rigidity. Other methods, such as histological examination, study by isotopical marking, vibratory methods and the use of electric voltages of the bone have not passed the experimental stage. However, none of these methods at present enable direct measurement of the evolution of the mechanical resistance of the bone, nor do they the determine the mechanical stability of the osteosynthesis assemblies during stresses or revalidation movements. It is also known to evaluate the reconsolidation by means of external fixation bars fitted with a single ohmic strain gauge. However, the use of a single gauge renders the method dependent upon the skill of the surgeon to direct this gauge in accordance with a maximal deformation plane as only one component of bending is determined. This technique only enables relative measurements of the evolution of the reconsolidation of the fracture.
On the other hand it has appeared that the rehabilitation of patients is slowed by the fear of causing a new fracture due to an overload exceeding the callus and fixation resistances, and simultaneously causing irreversible sliding in the clamps and movement of the bone fragments in the fracture site.
The object of the present invention is the objective determination of the torsion and of the bending of the external fixation bar at the level of the two closest clamps, situated on the bar, on either side of the gauged part.
It is thus possible to evaluate the risk of sliding in these clamps by comparing the determined stresses to the stresses capable of being borne by these clamps.